Principal neurons of the basilar pons as the source of a recurrent collateral system

The basilar pontine nuclei in the opossum are composed of two general categories of neurons, intrinsic cells and the principal or projection neurons. Observations from Golgi material indicate that principal neurons whose primary axons project to the cerebellar cortex may also give rise to recurrent branches distributing within the pontine gray. Such collaterals were observed to arise near the soma and at some distance from the cell body of the parent axon. The electron microscopic correlate of such a system was identified in the basilar pontine neuropil in animals subjected to lesions of the cerebellar cortex. These lesions destroyed mossy terminals and their parent axons and thus initiated a retrograde reaction in basilar pontine projection neurons which manifested itself in the form of morphologic alterations observed in somata, dendrites, and a class of axonal boutons. Similar altered axon terminals were not observed in control material and did not correspond to the terminals of cerebello-pontine axons described in previous work. It was therefore suggested that such boutons represented the terminals of the recurrent collateral system observed in Golgi material.